In the conventional practice of enhancing the heat insulating ability of castings, a piece of heat and corrosion resistant steel is sometimes inserted into a pipe after the latter has been cast. In this case, an air layer is formed between the casting and the steel lining or a heat insulator is inserted. According to another method, a ceramic core lining is provided and the entire assembly is enveloped with a casting to make it a double structure.
The former method is subjected to restrictions when the cast product has a complicated profile, while the latter method is not suitable for the cast product required to be durable and stable in quality, because of the weakness of ceramics to vibrations and heat shock under repeated cycles of heating and cooling.